Automatic test equipment for checking printed circuit boards has long involved use of a "bed of nails" test fixture to which the circuit board is mounted during testing. This test fixture includes a large number of nail-like spring loaded test probes arranged to make electrical contact under spring pressure with designated test points on the circuit board under test. Any particular circuit laid out on a printed circuit board is likely to be different from other circuits and consequently, the "bed of nails" arrangement for contacting test points in a particular circuit board must be customized for that circuit board. When the circuit to be tested is designed, a pattern of test points to be used in checking it is selected, and a corresponding array of test probes is configured in the test fixture. This typically involves drilling a pattern of holes in a probe plate to match the customized array of test probes and then mounting the test probes in the drilled holes on the probe plate. The circuit board is then mounted in the fixture, superimposed on the array of test probes. During testing, spring loaded test probes are brought into spring pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred from the board to the test probes and then to the exterior of the fixture for communication with a high-speed electronic test analyzer which detects continuity, or lack of continuity, between various test points in the circuits on the board.
Various fixtures have been used in the past for bringing the test probes and the circuit board under test into pressure contact for testing. One class of these fixtures is a wired test fixture in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the probes to the external electronically controlled test analyzer. These wired test fixtures are often referred to as vacuum test fixtures since a vacuum is applied to the interior of the test fixture housing during testing to compress the circuit board into contact with the test probes. In one type of vacuum test fixture, an upper probe plate supports the board under test and the test probes are mounted in a fixed lower probe plate. Holes are drilled in the upper probe plate to match the pattern of test points, and the test probes pass through the holes for contact with the underside of the board under test. During testing, a vacuum applied to the region between the upper and lower probe plates moves the upper probe plate toward the lower probe plate, which compresses the circuit board into contact with the test probes. This applies spring pressure from the probes to the test points on the board.
Various approaches have been used in the past for arranging the test probes in a pattern corresponding to the test points on the circuit board. A first approach called a grid type arrangement has an array of test probes placed in fixed locations independent of the circuit board under test. Rows of test points are electrically connected to the electronic test analyzer by printed circuit boards or wire harnessing. Rows of test probes are evenly spaced apart in a grid pattern. The test probes are aligned to corresponding test points on the circuit board using a panel containing holes that redirect the test probes to the closest test point. Thus, the basic grid-type fixture remains independent of the circuit board to be tested and is adapted to each circuit board by means of this panel. However, because the number of probes is independent of the number of test points needed, extensive wiring both within the test fixture and to the test electronics is required. Because extensive wiring is complex and costly, the major cost of this method of fixturing is in the grid fixture. Conversely, adapting this type of fixture to individual circuit boards is relatively inexpensive because the only change required is the addition of the panel with a hole pattern corresponding to the circuit board under test.
A second approach for arranging test probes uses the moving probe plate with test probes inserted in holes drilled in a pattern corresponding to the test points on the circuit board under test, as described above. This type of test fixture typically includes an electrical interconnect from the test probes to an interface connector for communicating the electrical test signals from the test probes to the electronic test analyzer. Under this approach, a unique test panel is required for each unique circuit board to be tested. In these custom type testers, fixturing is generally more complex than in the grid type fixtures, but with a custom system, the interfaces and test electronics are less complex and costly. It is the customized wired test fixtures to which the present invention is generally directed. The resulting cost advantages and fixturing make this type of test system more economical than the grid type system.
Various test probes have been used in the past. In one class of such probes, each probe contains a spring for applying a spring force against the test points. The typical spring probe comprises a tubular housing with a closed end and a compression spring inside the housing which bears against the closed end of the housing. The other end of the spring bears against the inner end of a movable plunger in the housing. When the spring is extended its desired length, the plunger engages a stop on the housing. During testing, the spring is compressed to apply a controlled spring force against the plunger so that a tip of the plunger applies a spring force to a test point on the circuit board. The tip of the plunger must be sufficiently small to achieve good electrical contact with the test point while avoiding contact with adjacent test points or circuit traces on the board. Similarly, the housing of each test probe must be small enough to not interfere with adjacent test probes, while maintaining the structural integrity of the test panel that holds the test probes.
As circuit board technology advances, circuit components are more closely spaced. The test points and probes for such circuits necessarily must be more closely spaced, requiring smaller probe and springs, which become more difficult and expensive to make.
In another class of test probes used in the prior art, such as shown in U.S. Pat. No. 4,340,858 to Malloy, each probe comprises an elongated solid metal pin mounted in a channel molded in a support panel. A circuit board closes the channels that hold the conductive pins. Electrical contact between the pins and circuits on the circuit board is achieved using individual spring clips which apply pressure to the moving pins. Longitudinal displacement of the pin in each channel is resisted by a compliant mat mounted in the base of the test fixture to engage ends of the pins. As the circuit board under test presses against the test probes, the compliant mat is compressed. Each pin thereby receives an opposite spring force to hold the pins against the board under test.
With this type of test probe, various compliant mats with different elastic properties must be used when different spring forces are required. Furthermore, for a given compressible mat, the spring force of the test probe against the circuit board is different for test probes with different displacements because the spring force provided by the mat varies with the distance of compression. For example, a test probe contacting a component on the circuit board will displace the compressible mat more than a test probe contacting a test point of the board. Thus, the spring force against the component is greater than the force against the test point. This can be troublesome for many types of components. Another disadvantage of the compressible mat is that it sometimes acquires a "set" after repeated use, which produces undesirable variations in the spring forces applied by the mat to the probes.
Some prior art pneumatic test fixtures apply a fluid pressure against one end of a test probe. Such a fixture is disclosed in U.S. Pat. No. 3,714,572 to Ham, et al., for example. The spring force applied by the test probe is limited by the transverse cross sectional area of the end of the probe. As test probes become smaller in diameter for higher density circuit boards, this test fixture applies less force to each probe.
A prior art test fixture using a flexible diaphragm is disclosed in U.S. Pat. No. 3,016,489 to Briggs. In Briggs '489, an electronic circuit card is mounted on rigid mounting channels for clamping the circuit card in place. Test probes are mounted opposite the circuit card in a metal assembly plate. The test probes have Teflon pegs that contact a flexible diaphragm at one end of the test probes. When the circuit card is mounted on the fixture, the probes engage the card and are pushed down against the flexible diaphragm. Hydraulic pressure is applied on one side of the flexible diaphragm to force the test probes into contact with the circuit card during testing. A metal cap is mounted at the end of each probe to contact the circuit card under test. A lead is soldered to an external test circuit terminal and soldered to the metallic conducting cap. For high density circuit boards that contain several thousand test points, the individual wiring of the leads to each probe can be time consuming, unreliable and costly. Furthermore, as test probes get smaller, as discussed above, the construction of test probes with multiple parts becomes increasingly more difficult and costly. The test procedure thus involves placing each board on the fixture by forcing it into pressure contact against the elastic resistance force of the diaphragm, removing the board after test and repeating the procedure for the next board. This procedure is not adaptable to the testing of boards rapidly and where the boards have high density test points.
A second prior art test fixture using a flexible diaphragm is disclosed in U.S. Pat. No. 4,232,928 to Wickersham. In Wickersham '928, test probes that contact the printed circuit board are connected to corresponding contact pins by wire wrap connections. The contact structures engage interface pins in a manner similar to that of the test probes. The pins are interconnected by wire wrap connections to wires for communicating test signals to the test electronics. A flexible diaphragm is spaced apart below a guide plate that holds the test probes and the contact pins. During testing the diaphragm engages the contact pins and the test probes and urges them into contact with the interface pins and the circuit board, respectively. The circuit board is mounted against a yieldable electrically nonconductive pad formed of a plastic foam material secured to the bottom of an enclosure. The foam pad applies force against the circuit card to counter the test probes and flex the diaphragm to create spring pressure on the probes. A vacuum applied above the diaphragm or a positive fluid pressure applied below the diaphragm maintains the diaphragm elastic force on the probes. For high density circuit boards that use thousands of test probes, the wire wrap connections between the test probes and the contact pins become time consuming, unreliable and costly. Furthermore, the numerous wired connections between the test probes and the contact pins provide an intertwined group of wires that will resist the spring force of the flexible diaphragm. Thus, for these high density circuit boards the diaphragm may not apply a proper spring force against the test probes and thus a reliable electrical contact between the test probes and the circuit board cannot be ensured. The diaphragm arrangement with respect to the probes and their means of support does not ensure direct uniform force applied by all probes to the board, due to lack of containment of the diaphragm under a pressure difference having more displacement in the center of the diaphragm than at the sides where the diaphragm is fixed.
During initial setup for test, contaminants, such as solder flux or foreign material, or oxidation on test probes or test points may prevent a test probe from achieving a reliable electrical contact with the test point. Some prior art test fixtures require the operator to release the circuit card from the test probes in an attempt to mechanically break the contaminants, electrical barrier. This requires the vacuum in pneumatic fixtures to be released, and, for all fixtures, power to be shut down. Reestablishing the vacuum and restarting power are costly because test time is increased.